The treatment of smoking-related disease is a major expenditure and there is a need for a convenient accurate determination of patient's smoking habit in order to determine appropriate and effective treatment of smoking-related diseases. There is also a demand for an assay device which can be used by non-chemists in extra-laboratory situations such as doctors surgeries, anti-natal clinics, industrial plants, water works, farms or the home. In order to be suitable for this, it is important for the assay device to be relatively easy and safe to use and to be relatively safely disposed of.